The operation of an implantable heart defibrillator requires the transmission of low level electrical signals generated by the heart to the defibrillator, as well as the pacing and defibrillation currents generated by the defibrillator to the heart. This physical link of transmission, the pacemaker or defibrillator lead, is a section of fatigue-resistant, insulated electrical conductor designed to endure the severe environment inside the human body. Any leakage of body fluids to the conductor can result in deterioration of the signals transmitted, due to corrosion of the conductor or connections, or to the creation of a current leakage path. The distal end of the lead, in the form of an electrode, provides conduction to the tissue. The proximal end of the lead in the form of a lead connector pin, is connected to the terminal of the defibrillator. The materials used to construct the pacemaker or defibrillator lead must be biocompatible. For insulation, common selections are silicone rubber and polyurethane. For the conductors, common selections are titanium and its alloys, 316L stainless steel, MP35N, silver composites, and platinum and its alloys.
Conventional pacemakers and defibrillators require a set screw to secure the lead connector pin to the terminal. A molded silicone rubber, thermoplastic, or epoxy top, or header, is used to encapsulate the terminal, leaving an opening, or connector cavity, for the lead connector pin to be inserted through. A small, self-sealing slit is cut in the silicone rubber top above the terminal to allow a wrench, such as an allen wrench, to gain access to the set screw. Alternatively, for an epoxy header, an additional opening is provided to the set screw, which is later sealed by application of silicone adhesive or a sealing cap screw. The set screw design provides good mechanical locking of the lead connector pin, along with electrical continuity between the lead connector pin and the terminal. A sealing mechanism on the lead connector located distal to the lead connector pin is used to prevent body fluids from contacting the lead connector pin and metal components of the terminal through the connector cavity.
During an implant operation, the surgeon is required to locate the external opening in the top, insert the wrench to engage the set screw, and apply the appropriate torque to tighten the set screw. There is a chance that the silicone rubber top or the self-sealing slit might become damaged and fail to provide adequate sealing, or that the cap screw or silicone adhesive may be inadvertently omitted or misapplied, thereby allowing body fluids to penetrate through to the terminal. This may result in deterioration of the heart signals, current shunting from the terminal to the subcutaneous tissue at the defibrillator generator implant site, and corrosion of the terminal and the lead connector pin. Even when properly applied, these extra sealing mechanisms required to cover the wrench entrance may be leaky over the life of the defibrillator.
The defibrillator is battery operated and therefore requires replacement every few years. However, the lead is still functional, and remains in place in the body. To replace the defibrillator, the set screw must be loosened to remove the lead. If the set screw, terminal, or lead connector pin has corroded, this may be impossible without damaging the lead.
Set screws and the wrenches required to use them are very small, are often supplied loose, and are difficult to handle with gloves. When used in the operating room, there is danger of their being dropped either outside the sterile field, or into the open wound of a patient where they may be difficult to retrieve.
An additional problem with prior art systems is that lead connector pins are often provided with a through hole for inserting a stylet to stiffen the lead to facilitate placement in the body. In that case, a set screw can collapse the wall of the pin, due to its point contact, deforming the pin and making it difficult to remove, or making it impossible to reinsert the stylet if necessary to move the lead. Such deformation is also undesirable since it could result in offset of the lead connector with respect to the connector cavity, resulting in poor sealing of the lead sealing mechanism. However, with a design using a balanced radial force on the lead connector pin, these problems would be minimized or eliminated.
New generation pacemakers and defibrillators require the terminals to be more compact in size and more robust in their use. The existing set screw design, limited by the use of a wrench and a threaded hole configuration, has been found to be inadequate not only with regard to compactness, but also with respect to ease of operation.
U.S. Pat. No. 5,069,209 to Posin describes a collet grip system, which eliminates the need for a set screw and other extra tools. This system uses a camming device to open and close the collet. The cams must be squeezed between the forefinger and thumb through the insulating header material. Therefore, this material is limited to a flexible material like silicone rubber. Also, current defibrillators typically have three or more lead connector cavities to accommodate at least two defibrillator leads and one pacing and sensing lead. The invention of Posin would not allow for side by side placement of the connector cavities, and would therefore require that the cavities be stacked to form an unusually tall header.
U.S. Pat. No. 4,540,236 to Peers-Trevarton describes a connector with a locking and releasing mechanism having gripping members for gripping a pacing lead connector pin having a specially formed groove. While the invention allows for easy locking and removal of a lead, the lead must be one having the special groove. Because few leads that are implanted have such a feature, the device would have limited use in the pulse generator replacement market. Also, two new international standards, ISO 5841-3, "Cardiac pacemakers--Part 3: Low-profile connectors (IS-1) for implantable pacemakers", and ISO 11318, "Cardiac defibrillators--Connector assembly for implantable defibrillators--Dimensional and test requirements", are approved industry standards. These standards specify a standard pacing and defibrillation connector assembly to allow leads and pulse generators from different manufacturers to be interchangeable. While it may be possible to use the invention of Peers-Trevarton to make a lead connector cavity conforming to the standards, the invention would not allow for the lead connector to conform to the standard, since there is neither a groove nor room for a groove on the pin in the standard. Since the lead connector cavity would require a lead with a groove in its connector pin, interchangeability with standard-conforming leads would be impossible.
U.S. Pat. No. 4,784,141 to Peers-Trevarton describes another lead locking mechanism which uses a compression ring instead of a collet or set screw to make contact with the lead connector pin. In this invention, the ring is tightened onto the pin by bearing down on it with a screw using a screw driver. The screw is sealed from the rest of the terminal by means of an o-ring that rotates and slides on the surface of a counterbore in the header.